Context-Based Entropy Coding of Sample Values of a Spectral Envelope

1. Parametric decoder configured to decode a spectral envelope of an audio signal by spectrotemporally predicting a current sample value of the spectral envelope to acquire an estimated value of the current sample value; determining a context for the current sample value dependent on a measure for a deviation between a pair of already decoded sample values of the spectral envelope in a spectrotemporal neighborhood of the current sample value; entropy decoding a prediction residual value of the current sample value using the context determined; and combining the estimated value and the prediction residual value to acquire the current sample value, and spectrally shape a spectrogram of the audio signal according to the spectral envelope, wherein the parametric audio decoder is configured to determine the spectrogram using spectral prediction and/or spectral entropy-context derivation.

2. Parametric decoder according to claim 1, further configured to perform the spectrotemporal prediction by linear prediction.

3. Parametric decoder according to claim 1, further configured to use a signed difference between the pair of already decoded sample values of the spectral envelope in the spectrotemporal neighborhood of the current sample value as to measure the deviation.

4. Parametric decoder according to claim 1, further configured to determine the context for the current sample value dependent on a first measure fora deviation between a first pair of already decoded sample values of the spectral envelope in the spectrotemporal neighborhood of the current sample value and a second measure for a deviation between a second pair of already decoded sample values of the spectral envelope in the spectrotemporal neighborhood of the current sample value, with the first pair neighboring each other spectrally, and the second pair neighboring each other temporally.

5. Parametric decoder according to claim 4, further configured to spectrotemporally predict the current sample value of the spectral envelope by linearly combining the already decoded sample values of the first and second pairs.

6. Parametric decoder according to claim 5, further configured to set factors of the linear combination so that the factors are the same for different contexts, in case of the bitrate at which the audio signal is coded being greater than a predetermined threshold, and the factors are set individually for the different contexts, in case of the bitrate being lower than the predetermined threshold.

7. Parametric decoder according to claim 1, further configured to, in decoding the sample values of the spectral envelope, sequentially decode the sample values using a decoding order which traverses the sample values time instant by instant with, in each time instant, leading from lowest to highest frequency.

8. Parametric decoder according to claim 1, further configured to, in determining the context, quantize the measure for the deviation and determine the context using the quantized measure.

9. Parametric decoder according to claim 8, further configured to use a quantization function in the quantization of the measure for the deviation, which is constant for values of the measure for the deviation outside a predetermined interval, the predetermined interval comprising zero.

10. Parametric decoder according to claim 9, wherein the values of the spectral envelope are represented as integer numbers and the length of the predetermined interval is smaller than, or equal to, 1/16 of the number of representable states of an integer representation of the values of the spectral envelope.

11. Parametric decoder according to claim 1, further configured to transfer the current sample value, as derived by the combination, from a logarithmic domain to a linear domain.

12. Parametric decoder according to claim 1, managing a number of contexts, each context having a probability distribution associated therewith which assigns to each possible value of the prediction residual value a respective probability, further configured to, in entropy decoding the prediction residual values, sequentially decode the sample values along a decoding order and use a set of context-individual probability distributions, which is constant during sequentially decoding the sample values of a spectral envelope.

13. Parametric decoder according to claim 1, further configured to, in entropy decoding the prediction residual value, use an escape coding mechanism in case the prediction residual value is outside a predetermined value range.

14. Parametric decoder according to claim 13, Wherein the sample values of the spectral envelope are represented as integer numbers, and the prediction residual value is represented as an integer number.

15. Method for parametric decoder comprising: decoding a spectral envelope of an audio signal by spectrotemporally predicting a current sample value of the spectral envelope to acquire an estimated value of the current sample value; determining a context for the current sample value dependent on a measure for a deviation between a pair of already decoded sample values of the spectral envelope in a spectrotemporal neighborhood of the current sample value; entropy decoding a prediction residual value of the current sample value using the context determined; and combining the estimated value and the prediction residual value to acquire the current sample value and spectrally shaping a spectrogram of the audio signal according to the spectral envelope, wherein the method comprises determining the spectrogram using spectral prediction and/or spectral entropy-context derivation.

16. Non-transitory computer-readable storage medium storing a computer program having a program code for performing, when running on a computer, a method according to claim 15.